Semiconductor devices and methods of making such devices

ABSTRACT

Single GaAs quantum well or single GaAs active layer or single reverse interface structures with Al x  Ga 1-x  As barrier layers have improved qualities when one or more narrow bandgap GaAs getter-smoothing layers, which are thin, are grown and are incorporated in the barrier layer before and in close proximity to the active layer.

This application is a continuation of application Ser. No. 831,473,filed Feb. 26, 1986 now abandoned, which is a divisional of applicationSer. No. 408,009, now U.S. Pat. No. 4,578,127.

TECHNICAL FIELD

This invention relates to semiconductor devices and methods of makingsuch devices.

BACKGROUND OF THE INVENTION

Many types of semiconductor devices have been investigated in recentyears in attempts to obtain, for example, devices with desirablecharacteristics such as enhanced carrier mobility. For example, U.S.Pat. No. 4,163,237 issued on July 31, 1979 to Raymond Dingle, Arthur C.Gossard, and Horst L. Stormer, describes a multilayered device having aplurality of alternate wide and narrow bandgap layers with modulateddoping, i.e., the wide bandgap layers are more heavily doped than thenarrow bandgap layers, and high mobility. The modulated doping permitsthe device, commonly termed a quantum well structure, to have electronmobilities which are higher than those in devices having uniformly dopedfilms. See Applied Physics Letters, 33, pp. 665-667, Oct. 1, 1978. Inone embodiment of such devices, the devices are fabricated with theAlGaAs/GaAs materials system.

Such quantum well structures, which, of course, need not have modulateddoping, have been extensively investigated. For example, multiple layerAlGaAs/GaAs quantum well structures that were grown by molecular beamepitaxy (MBE) have been shown to possess atomically smooth interfaces aswell as uniform layer thicknesses. See, for example, Applied PhysicsLetters, 29, pp. 323-325, Sept. 15, 1976. Photoluminescence spectra fromthese structures were predominantly intrinsic. See, for example, AppliedPhysics Letters, 38, pp. 965-967, June 15, 1981. This fact suggests bothhigh sample purity and luminescence efficiency.

However, several aspects regarding operation and characteristics ofthese devices have remained puzzling to workers in the field. Forexample, luminescence studies of some structures having a single GaAsquantum well showed extrinsic, impurity dominated luminescence as wellas nonuniformities in the layer thickness. Further, inequivalences wereobserved in the electrical transport properties of AlGaAs/GaAs singleinterfaces which apparently depended upon which layer was grown first.Additionally, in modulation doped single interfaces having the GaAslayer grown first, electron mobilities were observed which were higherthan those observed in multi-quantum well structures. See, AppliedPhysics Letters, 39, pp. 912-914, Dec. 1, 1981. In structures havingreverse single interfaces in which the AlGaAs layer was grown first,however, the electron mobility was either lower or no mobilityenhancement was observed. It should be further noted that it has beenreported that multiple quantum well structures, in which each quantumwell contains a reverse interface, show more mobility enhancement thanthe single reverse interface structures. See Journal of Applied Physics,52, pp. 1380-1386, March 1981.

SUMMARY OF THE INVENTION

We have found that a semiconductor device having a single semiconductoractive layer and a thin semiconductor cladding layer adjacent saidactive layer has desirable properties when a semiconductorgetter-smoothing layer is grown prior to growth of the cladding layer.In one embodiment, the active and getter-smoothing layers have narrowbandgaps and the cladding layer has a wide bandgap. The getter-smoothinglayer is typically between 10 and 10,000 Angstroms thick and theadjacent wide bandgap cladding layer is typically between 50 and 500Angstroms thick although it may be as thick as 1500 Angstroms. Thedevice, which may be, for example, a quantum well optical device, adouble heterostructure laser, or a field effect transistor, may furthercomprise other elements such as a second cladding layer or a bufferlayer between the substrate and getter-smoothing layer. In one preferredembodiment, the getter-smoothing layer and active layer materialcomprises GaAs and the wide bandgap cladding material comprises Al_(x)Ga_(1-x) As.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of a device according to this invention; and

FIG. 2 plots the binding energies of the neutral acceptors horizontallyin units of MeV versus the GaAs well width in units of Angstromsvertically.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown an illustrative embodiment ofour invention. For reasons of clarity, the elements of the device arenot drawn to scale. The device comprises substrate 1, and diverse layersincluding a first particle or light confining layer 3, i.e., a claddinglayer, a getter-smoothing layer 5, a second particle confining layer 7,i.e., a cladding layer, a single active layer 9, and a third confininglayer 11, i.e., a cladding layer. In one embodiment, layers 3, 7 and 11comprise wide bandgap materials and layers 5 and 9 comprise narrowbandgap materials. The active layer is the region where electron-holerecombination, light absorption, or carrier transport occurs. Thecladding layers confine light or carriers. The getter-smoothing layer 5is relatively thin, that is, it desirably has a thickness between 10 and100 Angstroms although greater thicknesses may be used. The secondparticle confining layer, i.e., cladding layer 7, is thin and desirablyhas thickness between 50 and 500 Angstroms although it may be as thickas 1500 Angstroms. The layers comprise a Group III-V compoundsemiconductor. In an illustrative embodiment, the narrow bandgap layerscomprise Al_(x) Ga_(1-x) As and the wide bandgap layers comprise Al_(x)Ga_(1-x) As, x greater than or equal to 0.0 and less than or equal to1.0. The substrate 1 and layer 11 are contacted electrically by meansthat are well known to those working in the art.

The devices of our invention are conveniently fabricated by molecularbeam epitaxy techniques. The growth of devices according to ourinvention by this technique is well understood by those working in theart and therefore need not be described in detail.

The device may be a quantum well optical device, a doubleheterostructure laser or a field effect transistor. The thickness of theactive layer will depend on the contemplated device application as iswell understood by those working in the art. For example, the quantumwell device will typically have a thickness between 20 Angstroms and 500Angstroms. Structural modifications of the embodiment depicted are alsocontemplated. For example, the getter-smoothing layer may comprise aplurality of narrow bandgap layers, i.e., a superlattice, and the secondcladding layer 11 may be omitted in some devices as may layer 3.

The critical nature of the thickness of the first, i.e.,getter-smoothing layer in structures having a single GaAs quantum wellbounded by Al_(x) Ga_(1-x) As barrier layers is shown by the following.Several structures were grown with different compositions in theunderlying layers. In particular, structures were grown on (100)oriented GaAs substrates under As₂ rich growth conditions. Thesestructures had (1) a single 100 Angstrom quantum well, i.e., activelayer, grown on a 1700 Angstrom Al₀.3 Ga₀.7 As cladding layer; (2) asingle quantum well grown on a similar Al_(x) Ga_(1-x) As cladding layerwith a single 10 Angstrom GaAs getter-smoothing layer 100 Angstromsbelow the 100 Angstrom quantum well; and (3) a single 100 Angstromquantum well with a single 50 Angstrom getter-smoothing layer 100Angstroms below the 100 Angstrom quantum well. All structures were grownwith a substrate temperature during growth of approximately 690 degreesC. and the growth commenced a 100 Angstrom GaAs layer followed by a sixlayer superlattice having 30 Angstrom layers of GaAs and 50 Angstromlayers of Al₀.3 Ga₀.7 As. The 100 Angstrom quantum well in eachstructure was capped with a 600 Angstrom Al₀.3 Ga₀.7 As layer.

Photoluminescence and photoluminescence excitation spectra were measuredat a temperature of approximately 6 degrees K. for these structures.Photoluminescence was excited at 1.737 eV with an intensity of 0.9W/cm². It was found that there was a strong increase inphotoluminescence intensity from the 100 Angstrom quantum well when thethin GaAs prelayer, i.e., getter-smoothing layer, was present below the100 Angstrom quantum well. Both the 10 Angstrom and 50 Angstromprelayers, i.e., getter-smoothing layers, improved the opticalproperties of the quantum well. However, much greater improvement wasobtained with the 50 Angstrom thick prelayer. The position of thequantum well luminescence line was identified as being that of anelectron to neutral acceptor recombination in a 100 Angstrom GaAs layer.The excitation spectrum of the quantum well luminescence was alsosharpest for the device having the 50 Angstrom thick prelayer and peaksat an exciting photon energy equal to the lowest intrinsic quantum wellexciton energy for a 100 Angstrom thick layer. This series of structuresshow that the first narrow bandgap layer should desirably have athickness between 10 and 100 Angstroms although layers with stillgreater thicknesses, i.e., up to 10,000 Angstroms, might be used.

An additional series of structures was grown which comprised (a) asingle 100 Angstrom thick GaAs quantum well layer grown on a 1 micronthick Al₀.3 Ga₀.7 As layer; (b) a single 100 Angstrom GaAs quantum welllayer grown on a thinner 200 Angstrom thick Al₀.3 Ga₀.7 As layer; and(c) a single 100 Angstrom thick GaAs quantum well grown on a micronthick superlattice of alternating 200 Angstrom thick Al₀.3 Ga₀.7 Aslayers and 200 Angstrom thick GaAs layers. Additionally, a GaAs bufferlayer having a thickness between 0.5 μm and 1.0 μm as well as a 600Angstrom thick Al₀.3 Ga₀.7 As cap layer were grown on all structures.The substrate temperature during growth of these structures was 690degrees C.

This series of structures demonstrates the effect of the thickness ofthe underlying Al_(x) Ga_(1-x) As layer, i.e., the second wide bandgapcladding layer, as well as the presence of the thin well superlattice onthe properties of the single quantum well layer of GaAs.Photoluminescence was excited at 1.710 eV with an intensity of 4.9W/cm². For the thick Al₀.3 Ga₀.7 As layer, i.e., structure (a), weakphotoluminescence occurred at the energy expected for electronrecombination with a neutral acceptor near one side of the quantum well.The excitation spectrum showed peaks at 100 Angstrom quantum wellexciton energies with widths corresponding to approximately 4 GaAsmonolayers of nonuniformity of the quantum well layer thickness. Withboth the thin Al₀.3 Ga₀.7 As layer and the superlattice, i.e.,structures (b) and (c), respectively, stronger and sharperphotoluminescence occurred with the photoluminescence from the 100Angstrom quantum well observed at the intrinsic quantum well freeexciton energy. This suggests higher layer purity. Excitation spectrapeaks were also sharper and more intense with widths corresponding tolayer thickness nonunformities of approximately one GaAs monolayer. Theinterfaces sensed by the excitons are therefore essentially atomicallysmooth. The AlGaAs cladding layer, i.e., the thin wide bandgap layer 7,should thus have a thickness less than approximately 1500 Angstroms.

These structures show that growth of GaAs layers or superlattices, i.e.,getter-smoothing layers, closely prior to a single GaAs quantum wellimprove both the smoothness and luminescent efficiency of the singlequantum well. The smoothest layers have the highest photoluminescenceintensity and the largest proportion of intrinsic luminescence relativeto impurity luminescence. It is hypothesized that this is because theGaAs near smooth AlGaAs/GaAs interfaces contain fewer impurities or elsethat the electrons at the smoother interfaces interact less stronglywith impurities that are near the interfaces. Evidence of the smoothingin the multi-quantum well sample has been seen by us in transmissionelectron microscopy cross-sectional images of multi-quantum wellstructures grown on initially rough surfaces.

We hypothesize that the first Al_(x) Ga_(1-x) As/GaAs heterojunction isoften characterized by a situation in which the first several tens ofAngstroms of the GaAs contain an acceptor-like impurity which producesextrinsic luminescence. Support for this hypothesis is found in FIG. 2in which values of the binding energy of neutral acceptor are plottedhorizontally in units of meV versus gallium arsenide layer thicknessvertically in units of Angstroms for nine different wafers. Also shownin FIG. 2 is a dashed line which represents data from undopedmulti-quantum well samples. The structures we grew had a p-type (100)GaAs substrate, a 1 μm GaAs buffer layer, 0.6 to 2.0 μm of Al_(x)Ga_(1-x) As, a single GaAs quantum well, and 0.5 μm of Al_(x) Ga_(1-x)As, and the data for these structures are indicated as circles. Theright and left solid lines represent theoretical calculations byBastard, Physical Review B, 24, pp. 4714-4722, 1981, for neutral carbonacceptors at the center of the well and at the Al_(x) Ga.sub. 1-xAs/GaAs interface, respectively. If the acceptors are uniformlydistributed in the GaAs wells, one would expect extrinsic luminescencepeaks at energies corresponding to acceptors at the center of the well.The present data suggest that there is a layer of GaAs at or near theinterface which is several tens of Angstroms thick and contains animpurity which is presumably carbon. Thus for wide wells, for example,150 Angstroms, the value is near that expected for acceptors near theinterface; on the other hand, for narrow wells, for example,approximately 50 Angstroms, the extrinsic layer essentially fills thewell so that the energy is more characteristic of impurities near thecenter of the well.

The origin of the impurity layer is not known with certainty but it ishypothesized to be caused by the following. The phenomena discussed canbe explained by an impurity, probably carbon, that is pushed ahead inthe growth direction as the Al_(x) Ga_(1-x) As/vacuum interface advancesand which degrades the interface smoothness due to its growth inhibitingnature. The impurity concentration builds up relatively slowly on theAl_(x) Ga_(1-x) As surface, i.e., hundreds of Angstroms will be grownbefore it reaches its maximum value. When the Al flux is terminated andthe GaAs getter-smoothing layer grown, the impurity is deposited in athin layer in the GaAs or perhaps desorbs from the surface. Thus, thewide bandgap layer 7 should be thin so that the concentration is low atthe interface between layers 7 and 9.

These results have significance for Al_(x) Ga_(1-x) As/GaAsheterostructure field effect transistor structures. There has beendifficulty in obtaining mobility enhancement in modulation dopedstructures grown by molecular beam epitaxy when the GaAs layer is on thetop. The impurity layers or interface roughness observed in somestructures described here may be connected with this problem.

What is claimed is:
 1. A semiconductor heterostructure device comprisinga layered structure on a substrate, said structure being adapted forelectron-hole recombination, light absorption, or carrier transport in asingle, homogeneous semiconductor layer which comprises GaAs and whichhere is designated as active layer, said structure further comprisingafirst semiconductor cladding layer which comprises Al_(x) Ga_(1-x) As,said first cladding layer being adjacent to said active layer and havinga thickness less than 1500 Angstroms, a semiconductor layer whichcomprises GaAs and which here is designated as getter-smoothing layer,said getter-smoothing layer being adjacent to said first cladding layer,and said getter-smoothing layer having a thickness in the range from 10to 100 Angstroms, and a second semiconductor cladding layer whichcomprises Al_(x) Ga_(1-x) As, said second cladding layer being betweensaid substrate and said getter-smoothing layer.
 2. The device of claim1, said active layer and said getter-smoothing layer having narrowbandgaps and said first and second cladding layers having wide bandgaps.3. The device of claim 1, said first cladding layer having a thicknessin the range from 50 to 500 Angstroms.
 4. The device of claim 1, saidstructure further comprising a third semiconductor cladding layeradjacent to said active layer on the side opposite said substrate.